Long base, stretched fiber-optic Bragg network extensometer and production method for same

ABSTRACT

An extensometer including a base, a tensioned optical fiber, and a Bragg grating. The extensometer relates particularly to monitoring of structures. Part of the optical fiber containing the Bragg grating is arranged in a tube. This part is tensioned between the two ends of the tube. The ends of this part are fixed to the ends of the tube, and the tube is rigidly fixed to a host material.

TECHNICAL FIELD

This invention relates to an extensometer using an optical fiber thatcomprises at least one Bragg grating and a process for manufacturingthis extensometer.

It is becoming more and more necessary to inspect aging of bridges, andmore generally of a large number of civil engineering works andinfrastructures. Most such concrete and steel structures are subjectedto severe environmental conditions (resulting from the climate, chemicalattacks and continuously increasing traffic) that explain theirpremature aging. Therefore, precise and reliable monitoring means,obviously with a life comparable with the life of the structures beingmonitored, must be provided for maintenance and public safety reasons.

Several physical parameters related to aging or degradation of thesestructures are concerned. For example, there are measurements of the pH,chemical composition, deformation and strain in the materials from whichthese bridges are composed, and detection and monitoring of cracks.

Within the context of this invention, we are particularly interested inmeasurements of deformations and detection and monitoring of thecracking state of a structure.

Measurements of deformations and elongations of large structures likethose encountered in civil or geotechnical engineering (for examplemines and building sites) are now made using various sensors, forexample extensometers with vibrating cords or inductive extensometersinstalled directly on these structures. Other contact free techniques(for example using ultrasound or optical means operating with visiblelight or lasers) are also known.

Note that conventional instruments (sensors and also the associatedelectronic means) installed on structures are frequently affected bylightning. The latter damages them and makes then inoperative, or evenirreplaceable, if these sensors and electronic means are embedded in themonitored structures.

Let us mention here and now that this invention proposes to use themetrological properties of Bragg gratings that can be written in thecore of optical fibers, preferably single mode fibers, and then to adaptthese fibers by the use of mechanical means and original attachmentpoints, so that they are used as extensometers in various domains,particularly civil engineering, public works and geotechnicalengineering.

Therefore, this invention proposes an innovative extensometer that canbe used to make tension and compression measurements and for thedetection and monitoring of cracks.

Applications and uses of this invention relate to extensometry in civilengineering, public works and geotechnical engineering, for example tomonitor road or railway bridges or viaducts, dams for hydroelectricpower stations, nuclear reactor buildings and cooling towers associatedwith these reactors, miscellaneous buildings, tunnels and mines, rockmovements and ground movements, or to check land or submarine seismicareas, buried pipes, pipelines, “riser” pipes, dikes and offshoreplatforms.

STATE OF PRIOR ART

It is known how a Bragg grating can be photo-inscribed in the core of aphoto-sensitive and generally single mode optical fiber. This Bragggrating consists of a spatial modulation of the optical index of thecore of the fiber for which the period Λ, in other words the pitch ofthe grating, defines the spectral wavelength of filtering intransmission, called the “Bragg wavelength” and denoted λ_(B), accordingto the phase match that forms the Bragg condition and that is written:λ_(B)=2·n _(eff)·Λ  (1)where n_(eff) is the effective index of guided mode.

When the Bragg grating is deformed, the Bragg matching condition issatisfied at a different wavelength. The spectral signature of the Bragggrating is modified, as a function of an elongation according to a firstorder linear law, and it satisfies the following equation:$\begin{matrix}{\frac{\Delta\quad\lambda_{B}}{\lambda_{B}} = {{\left( {1 - p_{e}} \right)ɛ} = {\left( {1 - {\frac{n^{2}{eff}}{2}\left( {{p_{12}\left( {1 - v} \right)} - {p_{11}v}} \right)}} \right)ɛ}}} & (2)\end{matrix}$where ε is the longitudinal deformation along the axis of the opticalfiber that is usually made of silica, p_(e) is the photo-elasticconstant of silica, p₁₁ and p₁₂ are elasto-optic coefficients and ν isthe Poisson's ratio of silica.

In extensometers conform with this invention, the properties of Bragggratings and optical fibers have properties as follows:

-   -   the use of optical fibers makes extensometers insensitive to        electromagnetic disturbances, lightning or electromagnetic        variations that do not have any effect on light transmission,    -   the intrinsic properties of Bragg gratings in terms of        transmission or spectral reflection are stable with time in an        environment like buildings and public works,    -   Bragg gratings remain stable at high temperatures of several        hundred °C.,    -   they may be spectrally multiplexed, in series or in various        network architectures or typologies, so that new extensometers        can be connected to the acquisition and measurement systems,    -   when they are used with instruments to make spectral        measurements, deformations are independent of the power of the        signal and any losses on measurement lines (for example due to        poor reproducibility of connections and curves in the optical        fibers), which is an important advantage in making long term        monitoring measurements.

Various extensometers that use optical fibers on which Bragg gratingsare placed are already known.

Normally, Bragg gratings are glued onto supports acting as proof bodies,or directly onto concrete reinforcing bars or prestressing cables. Metalproof bodies with this type of instrumentation can be embedded inconcrete.

Further information about this subject is given in the followingdocument:

-   -   [1] V. Dewynter-Marty, S. Rougeault, P. Ferdinand, D.        Chauvel, E. Toppani, M. Leygonie, B. Jarret and P. Fenaux,        Concrete strain measurements and crack detection with        surface-mounted and embedded Bragg grating extensometers,        12^(th) International Conference on Optical Fiber Sensors, 28-31        October 1997, USA, pages 600-603.

Bragg gratings may be embedded within composite bars, based on the samesolid transducer principle.

For example, further information about this subject is given in thefollowing document:

[2] Extensomètre à réseau de Bragg et procédé de fabrication de cetextensomètre, invention by S. Magne, V. Dewynter-Marty, P. Ferdinand andM. Bugaud, French patent application No. 99 04084, Apr. 1, 1999corresponding to International application PCT/FR00/00806, Mar. 30,2000.

Means of questioning Bragg gratings are also known and can be used withthe extensometer according to this invention. These means are based onspectral demultiplexing. Measurement line spectra (if the sensors aremultiplexed) are obtained using components such as Fabry-Perrot scanningcavities and can be used to analyze spectral offsets of Bragg gratingsat different frequencies, and thus use their sensitivities to calculatevariations in the physical parameter(s) considered (for exampledeformations, temperatures or pressures).

Means which use spectral filters for monitoring Bragg grating sensorsare also known.

There is a disadvantage with known extensometers: due to the structureof these extensometers, their base length cannot exceed more than abouta meter.

PRESENTATION OF THE INVENTION

The purpose of this invention is to overcome the disadvantage mentionedabove by proposing an extensometer that comprises an optical fiberprovided with at least one Bragg grating, the measurement base for thisextensometer being between a few tens of centimeters to a few meters, oreven a few tens of meters.

More precisely, the purpose of this invention is an extensometercomprising:

-   -   an optical fiber, in which at least one Bragg grating is formed,        and,    -   at least one proof body that is intended to be rigidly fixed to        a host material and that surrounds part of the optical fiber        containing this Bragg grating,    -   any deformation of the host material being thus transmitted to        this Bragg grating through the proof body, this Bragg grating        then being capable of modifying a light propagating in the        fiber, the deformation of the host material being determined        from this modified light,        this extensometer being characterized in that the proof body        comprises a tube in which the part of the optical fiber        containing the Bragg grating is placed, the two ends of this        part being fixed to the corresponding two ends of this tube and        this part being tensioned between these two ends of the tube.

According to a preferred embodiment of the extensometer according to theinvention, the proof body also comprises two end pieces fixed to thecorresponding two ends of the tube and the part of the fiber istensioned between these two end pieces.

According to one particular embodiment of the extensometer according tothe invention, the two end pieces are metallic, the optical fiber ismetallized and the two ends of the part of this fiber are soldered tothe corresponding two end pieces.

According to a second particular embodiment, the two ends of the part ofthe fiber are glued to the corresponding two end pieces.

Preferably, the two ends of the tube are soldered to the correspondingtwo end pieces.

Preferably, a metal tube and two metal end pieces are used and the twoends of the tube are soldered to the corresponding two end pieces by atin solder.

In a first particular embodiment of the invention, the proof body isintended to be embedded in the host material and the two end pieces areprovided with two corresponding means of anchorage into the hostmaterial.

According to a second particular embodiment, the proof body is intendedto be fixed to the surface of the host material and the two end piecesare provided with two corresponding means for attachment or gluing tothis surface of the host material.

In one particular embodiment of the invention, the extensometercomprises a plurality of proof bodies mounted in series.

This invention also relates to a process for manufacturing theextensometer according to the invention, in which the part of theoptical fiber between the two ends of the tube is tensioned and the twoends of this part are fixed to the two ends of the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be better understood after reading the descriptionof example embodiments given below, for information purposes only and inno way limitative, with reference to the appended drawings, wherein:

FIG. 1 is a diagrammatic longitudinal sectional view of a firstparticular embodiment of the extensometer according to the invention,

FIG. 2 is a diagrammatic longitudinal sectional view of a secondparticular embodiment of the extensometer according to the invention,

FIG. 3 diagrammatically illustrates networking of several extensometersaccording to the invention (four extensometers in this example), and

FIG. 4 diagrammatically illustrates an example of a device forinstalling an extensometer according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Extensometers according to the invention that are diagrammatically shownin longitudinal cross section in FIGS. 1, 2 and 3, will be used tomeasure deformations (contractions or expansions) of a host materialwhich in these examples is a concrete structure.

Each of these extensometers comprises an optical fiber containing atleast one Bragg grating, and the fiber containing the Bragg grating ispretensioned with an elongation greater than the maximum allowablecompression, so that elongation and contraction deformations can bemeasured.

Thus for concrete, the compression range is of the order of 2500 μdeformation (0.2%). It can be seen that it is quite possible topretension the fiber to more than this value (for example 0.5%).

FIGS. 1 and 2 show the concrete part 2 of this structure for whichdeformations are to be studied using one of these extensometers.

Each extensometer comprises a preferably single mode optical fiber 4,for example made of silica.

At least one Bragg grating is formed in the core of this optical fiber(by photo-inscription) (only one Bragg grating R in FIGS. 1 and 2, andfour Bragg gratings R1, R2, R3 and R4 in FIG. 3).

FIGS. 1 to 3 show measurement means 6 used to “question” the Bragggrating(s) and measure the deformations applied to the concrete.

These measurement means are designed to send light with a givenwavelength into the optical fiber 4 when there is only one Bragggrating, and light with different wavelengths when there are a pluralityof Bragg gratings (for example one wavelength for each Bragg grating).

Any deformation of the concrete is transmitted to a Bragg gratingthrough the proof body 8 (FIGS. 1 and 2) included in the extensometerand that we will describe in more details later.

This Bragg grating then modifies the light that corresponds to it andthis light returns to the measurement means 6 through the optical fiberand concrete deformation is determined by these measurement meansstarting from the light thus modified.

In the example shown, the measurement means 6 are connected to a singleoptical fiber, namely optical fiber 4 on which the Bragg grating R isinstalled.

Nevertheless, the measurement means 6 may comprise an optical switch toaddress one among N optical fibers (N>1), so as to connect the addressedfiber to the measurement means 6, at a given moment. Therefore, thisoptical switch provides a means of alternating the query (send andretrieve light) from a Bragg grating or lines of Bragg grating sensors,this query possibly taking place alternately from one end of the opticalfiber and then from the other end of the same fiber, and so on as shownin FIG. 3. This embodiment makes the system with one or severalextensometers more reliable, enabling this system to remain operationaleven if the fiber is accidentally cut.

However, in a simpler embodiment (not shown), the Bragg grating(s) canbe questioned from a single end of the optical fiber, the other endbeing free, in other words not connected to the extensometer.

In another embodiment (not shown), light can be sent into the Bragggrating(s) through one end of the optical fiber and this light can berecovered, possibly modified, from the other end of the optical fiber,to measure deformations of the concrete.

Now we will return to FIGS. 1 and 2. The proof body 8 of theextensometer according to the invention as seen in FIGS. 1 and 2comprises a central tube 16 and two end pieces 18 and 20 fixed to thecorresponding two ends of this tube.

The part of the optical fiber 4, in the core of which the Bragg gratingR is formed, is free inside this tube 16 and is tensioned inside thistube.

Another fiber, not shown and not tensioned, may possibly be attached tothis fiber 4 so as to measure the temperature.

The two ends of the part of the fiber on which the Bragg grating islocated are fixed to the inside of the corresponding two end pieces 18and 20, and this part of the fiber 4 is tensioned between the two endpieces.

This fiber is tensioned when the extensometer is being made, so that theextensometer can be used to make deformation measurements in tension andin compression along its longitudinal axis (axis of tube 16).

When one end piece moves relative to the other end piece, the Bragggrating is subjected to a measurable deformation.

There are three possibilities for attaching the part of the fiber 4located in the tube:

-   -   1) If the fiber is metallized in advance (in a known manner),        this part can be soldered in the end pieces.    -   2) If the fiber is provided with only one mechanical jacket or        protective jacket, usually made of polymer, or if it is not        provided with such a jacket, the part of the fiber can be glued        in the end pieces.    -   3) In another particular embodiment (not shown), a mechanical        clamping or capstan system can be provided for attachment of        part of the fiber 4 in the tube 16.

It is possible to choose the stiffness of the proof body 8 and thereforethe stiffness of the tube 16 as a function of the host material 2 inwhich this tube will be fitted.

The tube 16 is metallic, for example made of nickel-plated stainlesssteel, and is soldered at its two ends to the two end pieces also madeof stainless steel in the example considered. The essential function ofthis soldering is to maintain pretension of the fiber when theextensometer is put in storage and is put into place in the structure tobe monitored.

The fiber pretension can vary from a few grams to a few kilograms anddepends on the required measurement range.

The mechanical strength of the soldered connections between the tube andthe end pieces can be varied, as a function of the area to be solderedand the quantity of added material.

The stiffness of the extensometer may be low, to avoid introducingconstraints to deformation of the structure, or it may be more rigid,for example so that its equivalent Young's modulus is very similar tothe Young's modulus of the host material. This is done using a“flexible” solder, preferably a solder like that used in electronics.

The low stiffness of the extensometer tube 16 enables the use of longlengths for this extensometer, varying up to a few meters or even more.

All that is necessary is to adapt the length of the central tube 16(note that this type of measurement base is not available with vibratingcords for which the limit of the measurement base is now about 20 cm),while maintaining low intrusiveness (unlike “solid” extensometers).

“Solid” or “rigid” extensometers for which the Bragg grating is fixed toa metallic rod over its entire length, firstly have greater resistanceto deformations of the structure to be monitored and consequently arevery intrusive, and secondly they have to be sized to prevent the rodfrom buckling during operation in compression; as the length of themeasurement base increases, the diameter of the rod also increasesaccording to the basic principles of the strength of materials.Therefore it would be difficult to use these “rigid” extensometers tomake measurement bases more than about twenty cm long.

On the contrary, measurement bases with an extensometer according to theinvention are typically between about 0.1 m and several meters. All thatis necessary is to adapt the length of the central tube 16.

Several adaptations of the basic configuration that has just beendescribed (central tube 16, fiber 4 with Bragg grating R and two endpieces 18 and 20) are possible, enabling different applications.

In the example in FIG. 1, the extensometer is installed in a structureduring construction of this structure or after coring.

In this case, an anchor flange 22 or 24 is added to each end piece 18 or22, the anchor flange being screwed onto this end piece using two nuts26-28 or 30-32 that trap the corresponding flange through two washers34-36 or 38-40, as can be seen in FIG. 1.

In the example in FIG. 2, the extensometer is installed on the surfaceof an existing bridge made of concrete 2. In this case, the end pieces18 and 20 are fixed to the corresponding jumpers or clamps 42 and 44through two nuts 46-48 or 50-52 trapping this jumper or clamp.

In the case shown in FIG. 1, the two flanges define the deformationintegration base.

In the case shown in FIG. 2, the two jumpers or clamps define thisintegration base.

In FIG. 2, each jumper or clamp is provided with holes 54 designed forattachment to the bridge using the appropriate means such as screwssymbolized by chain dotted lines 56.

Instead, the two clamps could be bonded to the surface of the bridge.

Note that in the examples in FIGS. 1 and 2, the end of the fiber 4 thatis shown as not being connected, could be free or it could be connectedto another extensometer, or even connected to the measurement means 6.

The example in FIG. 3 diagrammatically illustrates how severalextensometers according to the invention can be put in a network.

Four extensometers are used in this example, using the same opticalfiber 4 and comprising Bragg gratings R1, R2, R3 and R4 inscribed atdifferent wavelengths on this fiber 4.

The first three extensometers are placed in series with commonattachment points in pairs. The fourth extensometer is located furtheraway; an extension of the fiber 4 separates it from the other threeextensometers.

The result is an assembly that can be used to make distributedmeasurements.

In the example in FIG. 3, the three extensometers in series use threeproof bodies comprising three tubes with different lengths.

One of these tubes, forming a central tube 58, is connected to the othertubes 60 and 62 through two central end pieces 64 and 65. These othertwo tubes are also provided with two terminal end pieces 68 and 66 attheir ends furthest from the tube 58.

The fourth extensometer uses a tube 63 provided with two end pieces 67and 70 at its ends.

The optical fiber 4 passes through the end pieces 68, 64, 65, 66, 67 and70 and the tubes 60, 58, 62 and 63. One of the Bragg gratings R1 islocated between the terminal end piece 68 and the central end piece 64.The Bragg grating R2 is located between the two central end pieces 64and 65, and the third Bragg grating R3 is located between the centralend piece 65 and the terminal end piece 66. The fourth Bragg grating R4is located between the end pieces 67 and 70. The optical fiber istensioned between adjacent end pieces 68-64, 66-66 and 67-70 and isfixed in these end pieces.

In one example (not shown), two identical extensometers are mounted inseries, corresponding to two measurement bases each 0.5 m long. Twotubes with a common end piece are then used, while an end piece isplaced at each of the other two ends of these tubes. An optical fiber onwhich two Bragg gratings are installed is then used and this fiber istensioned in the two tubes, between the end pieces.

The deformation measurement range is defined for each extensometer byapplying and controlling the prestressing. For example, the pretensionnecessary for an extensometer to operate in compression within adeformation range varying from −1000 μm/m to −3000 μm/m is typicallymore than 300 g for a silica fiber with a diameter of 125 μm.

Consequently, the measurement range is adaptable, particularly incompression, as a function of the pretension applied when theextensometer according to the invention is being manufactured.

We will now consider a process for manufacturing an extensometeraccording to the invention. We will firstly describe an example of anextensometer assembly bench. This assembly bench provides a means ofmaking extensometers with excellent metrological reproducibility,particularly for application of prestresses necessary for compressionmeasurements.

This example of the installation bench is diagrammatically shown in FIG.4 and comprises an assembly rod 72, plates 74 and 76, anchor end pieces78 and 80, a pulley 82 and a pulley support 84, and each anchor endpiece 78 or 80 comprises an anchor support 86 or 88 and an anchor cover90 or 92, and a plate 94 can also be seen under the pulley support base.The spindle of this pulley is marked with reference 96.

All these parts are assembled as can be seen in FIG. 4.

The assembly rod 72 supports the assembly and keeps the variouscomponents coaxial as a function of the chosen measurement base.

The plates 74-76 or 94 support anchors or the pulley support. Theseplates are fixed under the assembly rod 72.

The end pieces of extensometers are placed in the two anchor supportsduring assembly.

The optical fiber is put into position around the pulley ready forpretensioning.

We will now describe how an extensometer is made according to theinvention.

This example relates to manufacturing of a basic extensometer comprisingan optical fiber containing a single Bragg grating, one tube and two endpieces.

In step 1, the two metallic end pieces are cleaned. These end piecesdegreased for this purpose, for example using ethane trichloro 111.

In step 2, the outside surface of the optical fiber is cleaned. This isdone by carefully cleaning this optical fiber with acetone on theportions on which it is to be glued or soldered.

In step 3, areas that will not be glued or soldered in the end piecesare identified and marked on the fiber. If the measurement base of theextensometer is denoted L_(b) (FIG. 1), and the length of each end pieceis denoted L_(e) (FIG. 1), two marks will be made on the fiber atdistances L_(b)/2+L_(e)+1 cm on each side of the position of the Bragggrating. These marks will help to correctly position this Bragg gratingat the center of the tube.

In a step 4, the various elements are slid onto the fiber. Theseelements, namely the left end piece, the tube and the right end piecewith the threads facing outwards, are slid onto this fiber, in thisorder.

In a step 5, the assembly bench is adapted to the measurement base L_(b)of the extensometers to be made and the two plates 74 and 76 are putinto place fixed to the anchor supports on two heating plates (notshown).

In a step 6, the separate elements of the extensometer are positioned onthe assembly bench. The two end pieces are placed in the correspondingsupports of the installation bench and the extensometer tube is adjustedso that this tube is not stopped due to contact in an end piece. Thefiber is slid so that the two marks on this fiber can be seen on theoutside of the two end pieces. The right end piece is the end piece thatis near the pulley.

Subsequent steps are different depending on whether or not the opticalfiber is metallized.

The gluing procedure corresponding to steps 7a to 15a described beloware applicable for an optical fiber that only has a polymer coating. Wewill describe the end of the process for a metallized fiber later on,and in this case the corresponding steps will be 7b to 9b.

Step 7a is a step for preparation of the adhesive that will be used forgluing. After mechanical strength tests and measurements to confirm lackof creep under load at different temperatures, we chose a dual-componentepoxy glue as the adhesive to glue the fiber into its end pieces. Thetwo components of this adhesive, namely the resin and the hardener, areweighed and mixed in a previously cleaned dish.

Step 8a fluidizes the adhesive. This adhesive is heated to a temperatureof 40° C. to make it fluid, but without beginning polymerization. It canthus be drawn in using a syringe.

Step 9a consists of injecting the resin into the left end piece. Thisresin is injected into this left end piece through the hole 98 (FIG. 1)that is machined at the center of this end piece, from its surface.

Injection is stopped when resin begins to appear at the exit from theend piece.

Step 10a is a resin polymerization step. An anchor cover is placed onthe end piece to achieve good thermal conductivity. A temperature probeis placed in the threaded part of the end piece and the temperature isincreased up to 105° C. for two hours, and then up to 175° C. for fourhours, and then annealing is done at 230° C. for 16 hours. Thepolymerization cycle depends on the resin used.

Step 11a is a step in which the metal tube is soldered into the left endpiece. This end piece and the tube are heated throughout, to the soldermelting temperature. A tin solder known for use in electronics is used.The solder is melted and deposited in the hole of the end piece machinedfor this purpose, and on the end of the tube.

Step 12a is a step in which the metal tube is soldered in the right endpiece. This procedure is the same as in step 11a.

Step 13a is a fiber pretensioning step. The right end of the fiber ispassed around the pulley, and a previously calibrated mass is suspendedfrom it, to pretension the fiber. For a conventional single mode fiberfor which the optical cladding diameter is 125 μm, a mass of a 100 gtypically induces a pretension of 1000 μm/m.

Pretension is applied in the following steps.

Step 14a consists of injecting resin into the right end piece. Theprocedure is the same as in step 9a.

Step 15a polymerizes the resin injected into the right end piece. Theprocedure is the same as in step 10a.

The basic extensometer is then complete, if the coating is a polymer.

If the fiber is metallized, the end of the manufacturing processconsists of steps 7b to 10b.

In step 7b, the fiber is soldered into the left end piece, and this endpiece is soldered into the tube, at the same time.

In step 8b, the fiber is pretensioned. The procedure is the same as instep 13a.

In step 9b, the fiber is soldered into the right end piece, and this endpiece is soldered into the tube, at the same time.

The basic extensometer with the metallized fiber is then complete.

Step 16, consists of putting the fiber protective cables 100 into place(FIG. 1) and is applicable regardless of whether or not the fiber ismetallized.

These protective cables 100 are slid into place to protect the fiberthat projects from each end of the end pieces. Each cable 100 is fixedto the corresponding end piece using a gland 102 screwed to the insideof the end piece onto the threaded part provided for this purpose.

Let us give additional information concerning the assembly of anextensometer using the assembly bench in FIG. 4. This FIG. 4 shows thatholes 104 are provided at a uniform spacing from each other along theentire length of the rod 72. These holes 104 are used to modify thedistance between two consecutive end pieces as a function of the lengthof the tube and the chosen measurement base.

1. Extensometer comprising: an optical fiber, in which at least oneBragg grating is formed; and at least one proof body configured to berigidly fixed to a host material and that surrounds part of the opticalfiber containing the Bragg grating, wherein any deformation of the hostmaterial is transmitted to the Bragg grating through the proof body, theBragg grating configured to modify a light propagating in the opticalfiber, the any deformation of the host material being determined fromthe modified light, and wherein the proof body comprises a tube in whichthe part of the optical fiber containing the Bragg grating is placed,two ends of the part being fixed to corresponding two ends of the tubeand the part being tensioned between the two ends of the tube. 2.Extensometer according to claim 1, wherein the proof body furthercomprises two end pieces fixed to corresponding of the two ends of thetube, and wherein the part of the optical fiber is tensioned between thetwo end pieces.
 3. Extensometer according to claim 2, wherein the twoend pieces are metallic, the optical fiber is metallized, and the twoends of the part of the optical fiber are soldered to the correspondingtwo end pieces.
 4. Extensometer according to claim 2, wherein the twoends of the part of the optical fiber are glued to the corresponding twoend pieces.
 5. Extensometer according to claim 2, wherein the two endsof the tube are soldered to the corresponding two end pieces. 6.Extensometer according to claim 5, wherein the tube and the two endpieces are metallic and the two ends of the tube are soldered to thecorresponding two end pieces by a tin solder.
 7. Extensometer accordingto claim 2, wherein the proof body is configured to be embedded in thehost material and the two end pieces are provided with two correspondingmeans for anchoring into the host material.
 8. Extensometer according toclaim 2, wherein the proof body is configured to be fixed to a surfaceof the host material and the two end pieces are provided with twocorresponding means for attaching or gluing to the surface of the hostmaterial.
 9. Extensometer according to claim 1, comprising a pluralityof proof bodies mounted in series.
 10. Process for manufacturing theextensometer according to claim 1, wherein the part of the optical fiberbetween the two ends of the tube is tensioned and the two ends of thepart are fixed to the two ends of the tube.